The logic behind 5 votes needed is community moderation. It is to avoid unilateral moderation.
Hinduism Stack Exchange is moderated by you.
This is what the review page says. So the users with privilege can cast their votes to reopen or close a question.
Now coming to the specific case, the question was not closed by my vote. It was closed by the binding vote of OP who asked the question was satisfied with the linked question and the answers written to it. Then they clicked "That solved my problem". So the point to be noted is the OP themselves was satisfied with the vote. It's also worth noting that duplicates are sometimes suggestions.
What is the relationship between Hinduism and atheism? contains a couple of questions about Atheism. It asks what is the relationship between Hinduism and atheism, it asks whether all Hindus follow atheism. One of the sub question asked in the body is as follows:
Can you also cite the Vedas or Upanishads which talk about atheism in Hindu religion?
The same was also asked in another question which I have voted to close. Do Nastika and Astika have direct translations in English?
I ask the users of the site to read the body of the question too because the title alone is not the question. The main question is asked in the body only. We still have questions with titled "Question regarding X", "Shloka regarding Pandavas" which do not give any detail or hint what is the question actually about. For that we look into the body which has an explanation. So the body of the closed question asks
Is there an accurate translation of the words in English ?
Also, what do they exactly mean as per Hindu scriptures ?
The direct translation of the words is given in the answers written to What is the relationship between Hinduism and atheism?. Be Happy's answer gives what is the definition of that according to different schools of Hinduism, cite how word Nastika is used and other answers also give meaning. We are free to post new answers to the question with a different point of view also.
This is when we can vote to close a question as a duplicate. From Stack Exchange faq How should duplicates be handled
Questions may be duplicates if they have the same (potential) answers. This includes not only word-for-word duplicates, but also the same idea expressed in different words.
There are many very closely related questions whose answers would be potentially same. In such cases, duplicate works as a suggestion. Many sites close very closely related questions and popular questions even though they'renot exact duplicates to avoid repetition of same answers. Even our site has such policy for some questions like "Death in family - does the marriage need to be postponed". In these cases, duplicate vote works as suggestion. If the OP is happy with the closure, question will be closed.
In this case, it's not only the same idea expressed, they even have same words "Hindu scriptures" in their bodies. The answers also cite scriptures, more importantly, OP accepted that question with their binding vote (if questions are entirely different and closed as dupe by OP themselves, we can take an effort to reopen but the questions are very closely related, old question covers new question. We should not only look at the accepted answer. Since there's still scope for new answers, we are not limiting points of view. As said already, we can post new answers.
Also refer to In what scenarios 'Community' user cast a close as duplicate vote?.
Can we clarify to the OP that their question is not yet closed and the duplicates are only suggestions?